Browsing by Author "Gomes, Megan"

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    Diatom-based reconstruction of the Holocene evolution of Lake St Lucia, South Africa
    (2016) Gomes, Megan
    Coastal waterbodies along the east coast of southern Africa evolved from fluvial origins that were slowly drowned by rising sea levels during the Holocene. The accumulation of sediment in these systems is relatively undisturbed, providing ideal sites from which longer term observations of palaeo-climatic variability over most of the Holocene period can be made. Lake St. Lucia, on the north coast of KwaZulu- Natal, is the largest estuarine lagoon in Africa and is widely regarded as one of the most important shallow water systems globally. Despite the importance of this system, little is currently know about the processes driving the long-term evolution of the lake. This study aimed to reconstruct the hydrological changes associated with the Holocene evolution of Lake St. Lucia using fossil diatoms. Analyses were performed on two sediment cores from the North Lake (15.6 m) and False Bay (15.9 m) basins of Lake St. Lucia. Age models, each based on eight radiocarbon dates, revealed continuous sedimentary records covering ~8300 cal. yr BP. A total of 150 samples were examined resulting in a total of 113 species recorded which were used to infer changes in environmental conditions based on their reported ecological preferences.
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    Geomorphic adjustment, sensitivity, and connectivity of the lower Mara River and its floodplain wetlands, Tanzania
    (2024) Gomes, Megan
    The Mara River in semiarid East Africa traverses one of the world’s most important wilderness conservation regions. Like many other dryland rivers, the Mara sustains wetland and terrestrial ecosystems that provide essential ecosystem services to local human and wildlife populations. However, the hydrological and geomorphological diversity and complexity of dryland rivers, including the Mara, makes comprehensive understanding of their development and prediction of future biophysical changes difficult, representing major challenges in fluvial geomorphology and wetland management. This thesis examines the key geomorphic controls that drive dryland river morphology and floodplain wetland formation, as well as the intrinsic processes that lead to channel and wetland adjustment, within the Mara system. Morphometric analysis based on satellite imagery mapping and discharge modelling is used to assess the valley-scale controls and hydrogeomorphic forms and processes of the river and its palaeochannels, as well as recent historical geomorphic adjustments (last 32 years) along the lower Mara River. The river and palaeochannels have marked downstream declines in channel capacity and changes in sinuosity due to declines in discharge and stream power, coincident with a reduction in valley confinement. While several large palaeochannels extended further downstream than the modern river, likely driven by higher-energy hydrological regimes in the past, the river now exhibits a nonequilibrium response to rapidly declining fluvial energy resulting in avulsions, channel diminution, and channel breakdown in the wetlands. Geomorphic sensitivity analysis of the lower Mara River for the recent historical period indicates that the upper and lower reaches are ‘Passive sensitive’, the former with limited adjustments, the latter with numerous cut-offs and palaeochannels, and both reaches having low migration rates. In contrast, the middle reach is ‘Fragile’ with numerous lateral migration features, cut-offs, and higher migration rates. The lower reach appears to be close to a threshold and may switch to a Fragile state where avulsions become dominant, while the other reaches may be more resilient and maintain their present states. These complex geomorphic responses are likely related to spatiotemporal patterns of hydrogeomorphic connectivity in the catchment. Modelling of contributary hydrogeomorphic connectivity indicates that the upper catchment (source zone) is consistently well-connected and is responsible for the greatest inputs of water and sediment to the fluvial network. The middle 4 section (transfer zone) has lower and more variable seasonal, inter-annual and decadal connectivity, while the lowland reaches (deposition zone), where large floodplain wetlands occur, have relatively consistent, low connectivity. These trends are associated with downstream sedimentation resulting in river and wetland adjustments and avulsive behaviour. Together, these findings provide novel insights into dryland river character and behaviour, as well as the valley- and catchment-scale conditions that contribute to geomorphic adjustments in internationally significant fluvial systems. Under increasing pressure from anthropogenic and climate change impacts the Mara River and its wetlands are likely to undergo future geomorphic changes, and thus an understanding of the biophysical controls, forms, and processes of river adjustment as well as states of sensitivity and patterns of connectivity are imperative for guiding conservation and management efforts.